1. Field of the Invention
This invention relates to a washing solution for washing the surface of a semiconductor substrate such as a silicon wafer and a washing method using the same.
2. Description of the Related Art
On the surface of this kind of a semiconductor substrate, metal impurities or fine particles having a particle size of 1 .mu.m or less are attached during the preparation process. Accompanying with high accumulation and high functionalization of a semiconductor device, it is more and more desired not to contaminated the surface of the semiconductor substrate by these metal impurities or fine particles, etc. Thus, washing technique of a semiconductor substrate is particularly important in the techniques of whole semiconductor devices.
As the conventional washing method of a semiconductor substrate, there has been known an RCA washing method using an SC1 solution comprising hydrogen peroxide and ammonium hydroxide, and an SC2 solution comprising hydrogen peroxide and a diluted hydrochloric acid. In the RCA washing method, a semiconductor substrate is firstly immersed in the SC1 solution to remove fine particles and organic residue from the substrate by the oxidizing and alkaline properties of the solution. That is, in the SC1 solution, both, reactions of oxidization and reduction are simultaneously carried out, and reduction due to ammonia and oxidization due to hydrogen peroxide occur in the same bath competitively, and simultaneously, fine particles are removed from the substrate surface by lifting off due to an etching effect of the ammonium hydroxide solution. Then, the semiconductor substrate is immersed in an acidic solution of the SC2 solution to remove alkali ions or metal impurities which are insoluble in the SC1 solution.
On the other hand, Japanese Laid-Open Patent Application No. 94458/1995 discloses a washing solution of a semiconductor substrate which prevents metal impurities in a washing solution from adhering to the substrate surface when the semiconductor substrate is washed or prevents metal impurities once removed from the substrate surface from adhering to the substrate surface again, and inhibits unnecessary growth of a naturally oxidized film, and a washing method using the same. This washing solution is constituted by an acidic solution such as hydrofluoric acid containing 0.0001 to 0.001% by weight of ammonia or 0.0005 to 0.01% by weight of ethylenediaminetetraacetic acid (EDTA). In this washing solution, pH thereof is maintained to 1 or so in order to remove an oxidized film. According to this washing method, when a semiconductor substrate 1 is immersed in a washing solution 5 as shown in FIG. 6, removal of a naturally oxidized film 2 and removal of metal impurities 4 are simultaneously proceed, and a metal element shifted over to the washing solution forms a complex, or a metal complex salt 7 whereby it is masked. The surface of the semiconductor substrate 1 is charged to minus in the acidic solution while the surface of the metal complex salt 7 is also charged to minus by forming a complex ion in the acidic solution so that the metal complex salt, i.e., a metal element is prevented from adhering again to the substrates
However, in the above-mentioned RCA washing method, metal impurities and fine particles which contaminate the substrate are removed by two baths so that washing steps are complicated such as washing of a chemical solution and washing accompanied thereby Also, two kinds or more of acid and alkaline solutions are required as chemical solutions for washing. According to the above, there are disadvantages that this washing method requires a longer washing time, a washing device is a large-sized and a washing cost becomes expensive
Also, according to the washing method described in Japanese Laid-Open Patent Application No. 94458/1995, as shown in FIGS. 6(c) and 6(d), the surfaces of fine particles 3 are charged to plus in a washing solution 5 of strong acid so that even when metal impurities 4 can be removed from the semiconductor substrate 1 by the reason as mentioned above, there is a disadvantage that fine particles 3 are adhered to the surface of the substrate 1 and cannot be removed.
Further, the above-mentioned RCA washing method occurs two reactions of oxidation and reduction in the same bath competitively. Thus, firstly, metal impurities liberated from the substrate surface is retained in the SC1 solution whereby they are sometimes adhered again to the substrate surface depending on the surface potential, and secondly, even when a metal complex salt is to be formed by complexing metal ions in the SC1 solution, an organic acid is subjected to oxidization-reduction treatment in the SC1 solution whereby its complexing effect is markedly lowered. Accordingly, there is a problem that metal impurities cannot sufficiently be removed by the RCA washing method depending on the kind of a metal.